1. Field of the Invention
The present invention relates to medical imaging of blood vessels, and more particularly concerns the use of magnetic resonance to obtain such images.
2. Description of Related Art
Angiography, or the imaging of vascular structures, is very useful in diagnostic and therapeutic medical procedures. MR angiography is performed with a variety of methods, all of which rely on one of two basic phenomena. The first phenomenon arises from changes in longitudinal spin magnetization as blood moves from one region of the patient to another. Methods that make use of this phenomenon have become known as "in-flow" or "time-of-flight" methods. A commonly used time-of-flight method is three-dimensional time-of-flight angiography. With this method, a region of interest is imaged with a relatively short repetition time, TR, and a relatively strong excitation radio-frequency (RF) pulse. This causes the MR spins within the field-of-view to become saturated and give weak MR response signals. Blood flowing into the field-of-view, however, enters in a fully relaxed state. Consequently, this blood gives a relatively strong MR response signal, until it too becomes saturated. Because of the nature of blood vessel detection with time-of-flight methods, the stationary tissue surrounding the vessel cannot be completely suppressed. In addition, slowly moving blood, and blood that has been in the imaged volume for too long, becomes saturated and is poorly imaged.
A second type of MR angiography is based on the induction of phase shifts in transverse spin magnetization. These phase shifts are directly proportional to velocity and are induced by flow-encoding magnetic field gradient pulses. Phase-sensitive MR angiography methods exploit these phase shifts to create images in which the pixel intensity is a function of blood velocity. While phase-sensitive MR angiography can easily detect slow flow in complicated vessel geometries, it will also detect any moving tissue within the field-of-view. Consequently, phase-sensitive MR angiograms of the heart have artifacts arising from the moving heart muscle and from the moving pools of blood in the heart chambers.
In conventional MR imaging, an inhomogeneity of the static magnetic field produced by the main magnet causes distortion in the image. Therefore a main magnet having homogeneity over a large region is desirable.
Also, a stronger static magnetic field created by the main magnet, yields a better the signal to noise ratio, all other factors being equal. Typically, the magnets used to create the static magnetic field in an MR scanner have been constructed of a superconducting material requiring very low temperatures, and related support apparatus. These magnets can be very expensive.
Even if a very high field magnet were constructed to maximize the signal-to-noise ratio for MR angiographic imaging, the signal of the surrounding tissue would be increased to the same extent as signals from blood. Consequently, there would be no increase in vessel visibility with the higher magnetic field (assuming all other factors are equal).
Currently, there is a need for a system for obtaining high quality angiography of a selected vessel without the problems incurred with a large high-field imaging magnet.